Gravitational waves: Stars as giant gravitational wave detectors

Comparison of present upper bounds on an SBGW and the limits deduced from helioseismic data of the Sun.

We present a method to directly detect or constrain a stochastic background of gravitational waves (SBGW) at mHz and Hz frequencies by using helioseismic observations. We demonstrate the validity of this approach by deducing a direct upper bound around 0.17 mHz on both the astrophysical and cosmological component of the SBGW.

The universe is expected to be penetrated by a stochastic background of gravitational radiation of astrophysical and cosmological origin. This background is not accessible to conventional observations based on electromagnetic waves. In recent years an increasing number of experiments have set upper limits on the amplitude of this background at various frequencies. However, the astrophysically interesting band between 10-4 - 1 Hz remains largely unexplored. As shown by Boughn & Kuhn (1984) the Sun and stars can be employed as giant hydrodynamical detectors for the gravitational-wave background. Using recent high-precision radial velocity data for the Sun and a new theoretical formalism we re-deduce a direct upper bound constraining the normalized energy density of the stochastic background to ΩGW < 4 x 105 at 0.17 mHz. Such results demonstrate that helio- and asteroseismology can probe fundamental physics. This opens up a new perspective for missions like CoRoT, Kepler, and PLATO as well as for global telescope networks like SONG and LCOGT providing long and uninterrupted high-precision photometric or radial velocity time series.

Comparison of present upper bounds on an SBGW and the limits deduced from helioseismic data of the Sun: CMB observations at large angular scales indicate an upper limit on the cosmological component of the SBGW at larger wavelengths than the horizon size at the time of decoupling (3 × 10−18 - 10−16 Hz; Maggiore 2000). Pulsar timing observations based on the residuals of the pulse arrival times yield upper bounds between 10−9 - 3×10−8 Hz (Jenet et al. 2006). An upper limit from Doppler tracking of the Cassini spacecraft is obtained in the frequency range 10−6 - 10−3 Hz (Armstrong et al. 2003). At 0.2 Hz the torsion-bar antenna TOBA finds ΩGW < 4.3 × 1017 (Ishidoshiro et al. 2011), and a pair of synchronous recycling interferometers has placed ΩGW < 1.2 × 1026 at 100 MHz (Akutsu et al. 2008). A cross-correlation measurement between the Explorer and Nautilus cryogenic resonant bar detectors yielded ΩGW < 122 at 907.2 Hz (Astone et al. 1999). Also indicated are the upper limits from the S1 to S5 science runs of the Earth-based interferometric detector LIGO around 100Hz, with the tightest bound being ΩGW < 7.3×10−6 between 41.5 - 169.25 Hz at 95% confidence (Abbott et al. 2009). Indirect bounds can be deduced from BBN and CMB data (Maggiore 2000; Cyburt et al. 2005; Smith et al. 2006), which constrain the integrated total energy density of the cosmological component of the SBGW in the indicated frequency ranges.

 

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